STRUCTURE AND PHYSIOLOGY OF ANIMALS 



31 



number of them may unite together to form a single mass of 

 protoplasm with many nuclei, termed a plasmodium. 



ax 



neur 



FIG. 21. Nerve-cells. A, multipolar ; 

 B, bipolar. 



Fia. 22. Nerve-fibres. A, raedullated ; 

 B, non-medullated. ax, neuraxis ; 

 med, medullary sheath ; neur,; 

 neurolemiua. 



The characteristic cells of the reproductive tissues are the 

 ova and the spermatozoa or sperms. The ova (Fig. 6), when fully 

 formed, are relatively large, usually spherical cells, sometimes 

 composed entirely of protoplasm, but usually with an addition of 

 nutrient food-yolk. Each ovum, 

 as already mentioned, encloses a 

 large nucleus (germinal vesicle) and 

 in the interior of that one or 

 more nucleoli or germinal spots. The 

 sperms (Fig. 23) are extremely 

 minute bodies, nearly always motile, 

 usually slender and whip-like, 

 tapering towards one extremity, and 

 commonly with a rounded head at 

 the other. The sperms are de- 

 veloped by a succession of cell- 

 divisions from certain cells the 

 primitive male cells - - similar in 

 character to immature ova. During 

 the course of this development 

 (spermatogenesis) there is, as in the 

 maturation of the ovum (p. 20), a reduction of the number of 

 chromosomes in each nucleus by one half. 



5. ORGANS. 



The chief systems of organs of an animal are the integumen- 

 tary, the skeletal, the muscular, the alimentary or digestive, the 



FIG. 23. Various forms of spermatozoa. 

 a, of a Mammal ; b, of a Turbellarian 

 worm ; c, d, and e, of Nematode 

 worms ; /, of a Crustacean ; g, of a 

 Salamander ; h, the commonest form 

 with oval head and long flagellum. 

 (From Lang's Comparative Anatomy.) 



